1. Field of the Invention
The present invention relates to electrophotographic photosensitive materials, more particularly to electrophotographic photosensitive materials exhibiting both excellent electric properties and mechanical strength for a prolonged period of time.
2. Description of Related Art
Recently, electrophotographic techniques have been significantly applied to the field of copying machines, laser beam printers and facsimiles because they provide the advantages of yielding high print qualities at high speeds. Materials for electrophotographic photosensitive materials which have widely used in the electrophotographic techniques include known inorganic photoconductive materials such as selenium, selenium-tellurium alloys, selenium-arsenic alloys and cadmium sulfide.
On the other hand, electrophotographic photosensitive materials comprising organic photoconductive materials have also actively been studied since they have excellent advantages in cost, producibility and disposability as compared with the known electrophotographic photosensitive materials using inorganic photoconductive materials. In particular, separatedly functional organic laminated photosensitive materials comprising a charge-generating layer which generates charge upon exposure and a charge-transporting layer laminated thereon are excellent in their electrophotographic properties such as sensitivity, chargeability and its repeating stability. Various proposals have been made on these materials and many of them have actually been put to practical use. On the other hand, single layer organic photosensitive materials have advantages such as high productivity, low production cost and those associated with positive charging (i.e., reduced generation of ozone, uniform chargeability); at present, however, their electrical performance is poorer than the laminated photosensitive materials. Thus, there is still plenty of room for research and development.
In these organic photosensitive materials, the above mentioned electrophotographic properties have been developed to sufficient levels of performance; however, durability to mechanical external forces is poor since they are composed of organic materials. For example, the image quality may be deteriorated by wear of and/or wounds on the photosensitive material surface generated due to direct load from toners, developers, papers and cleaning members and from recent rollers which are brought to directly contact with photosensitive materials to charge up, as well as by adhesion of foreign matters such as toner filming onto the surface. Under high humidity environment, blurring of images may be caused by deterioration of the surface layer due to ozone and/or nitrogen oxides generated by corona discharge and/or to paper dust from copy paper which is adhered to and accumulated on the surface of photosensitive materials. These problems determine the lifetime of photosensitive materials.
Upon digitization of copying machines or printers, the sensitivity of photosensitive materials should be maximized in the wavelength range (780 to 830 nm) of a near infrared semiconductor laser so as to be exposed to a semiconductor laser. Further, a cycle of process, i.e., charging, exposure, development, transfer, cleaning and charge removal, tends to be effected in a still shorter period of time as coloration, acceleration and miniaturization are advanced. Therefore, there is a need for faster light responsibility and longer electrical stability. Thus, higher durability is required in view of complication of processes and higher stress on the photosensitive material as well.
At present, charge transporting polymeric compounds have actively studied since they may obviate the above mentioned deficiencies to a large extent and can possibly meet the above mentioned requirements. For instance, U.S. Pat. No. 4,806,443 discloses polycarbonates polymerized from specific dihydroxyarylamines and bischloroformate. U.S. Pat. No. 4,806,444 discloses polycarbonates polymerized from specific dihydroxyarylamines and phosgene. U.S. Pat. No. 4,801,517 discloses polycarbonates polymerized from bishydroxyalkylarylamines and bischloroformate or phosgene. U.S. Pat. No. 4,937,165 and U.S. Pat. No. 4,959,288 disclose polycarbonates polymerized from specific dihydroxyarylamines or bishydroxyalkylarylamines, and polyesters polymerized from bisacylhalides, respectively. U.S. Pat. No. 5,034,296 discloses polycarbonates or polyesters of arylamines having a specific fluorene skeleton. U.S. Pat. No. 4,983,482 discloses specific polyurethanes. Further, Japanese Patent Application Publications (JP-B) No. 59-28903 and Japanese Patent Application Laid-Opens (JP-A) No. 53-87226 disclose polyesters having a specific bisstyryl bisarylamine as a main chain. Japanese Patent Application Laid-Opens (JP-A) No. 61-20953, No. 1-134456, No. 1-134457, No. 4-133065 and No. 4-133066 propose polymers having a charge transporting pendant substituent such as hydrazone or triarylamine and photosensitive materials using the same.
Further, Japanese Patent Application Laid-Open (JP-A) No. 6-21416 proposes photosensitive materials in which an antioxidant is added to a charge transporting polymeric compound for the purpose of improving reactive gas resistance and repeating stability as well as mechanical strength.
Although electrophotographic photosensitive materials having a relatively good durability may be obtained by using the above mentioned, previously proposed charge transporting polymeric compounds as photosensitive layers, they are still unsatisfactory in the following: Coatings formed from these charge transporting polymeric compounds do not necessarily have sufficient mechanical strength, such that when used repeatedly in a copying machine for a long period of time, the surface of a photosensitive material wears and therefore the film thickness of the photosensitive material changes, the charging voltage reduces and the sensitivity varies; as a result, a copy may be fogged or the copy density may be reduced. The image quality may deteriorate due to wear and wounds on the photosensitive material surface.
In the proposed combination with an antioxidant, the image quality may be deteriorated by discharging products such as ozone generated upon charging. Further, electrical properties are not always sufficient in the case in which the combination is applied to a high speed electrophotographic device having a small diameter drum.